The present invention relates to a film selectively permeable to carbon dioxide gas derived from a specified natural raw material and to a food packaging film comprising the same. More particularly, the present invention relates to a film selectively permeable to carbon dioxide gas which comprises chitosan that is basic polysaccharide and in which the ratio of the carbon dioxide gas transmission (CO2TR) to the oxygen gas transmission (O2TR), (CO2TR/O2TR), is high, is highly safe from the standpoint of food hygiene, is insoluble in water, and is suitable for packaging fermented foods and vegetables, cut flowers and the like, and to a food packaging film comprising the same.
In the field of food packaging, technologies related to films that are selectively permeable to carbon dioxide gas include, for example, a molded article for packaging food, which comprises a composition composed of a polyvinyl alcohol resin and at least one compound which is selected from the group consisting of an alkylene glycol monomer, a hydroxy acid monomer and polymers thereof and which has a ratio of the solubility of carbon dioxide to the solubility of oxygen gas being 30 or more as disclosed in JP-A-5-222215. This publication describes that the films obtained from the molded article are suitable for packaging those foods which generate much carbon dioxide gas, and disfavor contact of oxygen, for example, cheese products, coffee beans and so forth. JP-A-9-316208 describes that a film obtained by heat-treatment of a reaction mixture of a polyacrylic acid and an aliphatic diamine has a resistance to water and has a ratio of the permeability to carbon dioxide gas to the permeability to oxygen gas of 15 or higher. Further, JP-A-11-538 describes that a film that comprises a polymer having a polyamine structure has a high ratio of the carbon dioxide gas transmission to the oxygen gas transmission.
For such a material having selective gas permeability, a progress has been made in the development of applications in particular as food packaging materials, and there have been demanded packaging materials in view of satisfying the safety from the standpoint of food hygiene.
An object of the present invention is to provide in an economical way a film which is insoluble in water, is highly safe from the standpoint of food hygiene, and has such selective gas permeability that the ratio of the carbon dioxide gas transmission to the oxygen gas transmission is high.
The inventors of the present invention have found that a film, which comprises a specified chitosan, has a high ratio of the carbon dioxide gas transmission to the oxygen gas transmission and can solve such problems, thus achieving the present invention.
Therefore, the present invention provides a film selectively permeable to carbon dioxide gas, which comprises a chitosan having a degree of deacetylation of 70 mol % or higher and in which the ratio of the carbon dioxide gas transmission (CO2TR) to the oxygen gas transmission (O2TR), (CO2TR/O2TR), is 15 or higher. The present invention provides a film selectively permeable to carbon dioxide gas comprising a laminate film including at least three layers composed of an outer layer, an intermediate layer and an inner layer, in which the outer layer and the inner layer comprise a thermoplastic resin and the intermediate layer comprises a film according to the above-mentioned invention, and in which the ratio (CO2TR/O2TR) as the laminate film is 10 or higher. Further, the present invention provides a film selectively permeable to carbon dioxide gas according to the above-mentioned invention, in which the film composed of chitosan in the intermediate layer has a thickness of 0.2 to 50 xcexcm. Further, the present invention provides a food packaging film which comprises a film selectively permeable to carbon dioxide gas according to the above-mentioned invention, in which the food is coffee, cheeses, bean pastes, pickles, root crops, mushrooms, leaf-stem crops, fruit vegetables, or fruits.
Chitosan can be obtained by deacetylating chitin, which exists widely in nature, for example, as a component constituting shells of lobsters and crabs, in a concentrated alkali. Fully deacetylated chitin is a basic polysaccharide having two hydroxyl groups and one amino group in the constitutional unit molecule thereof. Free chitosan does not dissolve in water and organic solvents but forms a salt in the presence of an inorganic acid such as hydrochloric acid or organic acid such as acetic acid, propionic acid, oxalic acid, butyric acid, lactic acid, tartaric acid, succinic acid, or citric acid to give an acid salt solution. As for the chitosan used in the present invention, one that has a molecular weight of 10,000 to 1,000,000, and further 10,000 to 500,000 and a viscosity of 1 to 20,000 cp (1 to 20,000 mPaxc2x7s), and further 1 to 2,000 cp (1 to 2,000 mPaxc2x7s) is preferred in consideration of coating properties as shown in the examples described hereinbelow. The degree of deacetylation of chitosan is 70 mol % or higher, preferably 80 mol % or higher. It is desirable that the viscosity of chitosan is 3 to 2,000 cP (3 to 2,000 mPaxc2x7s) for preparing solutions. Note that chitosan having a degree of deacetylation of below 70 mol % obtained by treating chitin in a concentrated alkali does not dissolve in acetic acid and cannot form films.
When using chitosan, it is handled in the form of the aforementioned chitosan acid salt solutions in consideration of ease of handling. Among these, acetic acid salt and lactic acid salt are preferred in consideration of price and safety of foods. In the chitosan acid salt solutions, hydrophilic organic solvents, for example, alcohols such as methanol, ethanol and isopropanol and ketones may be used as diluents. Further, when coating a chitosan acid salt solution on a substrate, in cases where the chitosan acid salt solution is difficult to coat on the surface of the substrate and thus being repelled, use of the diluent or of a small amount of surfactant will make the coating easy.
To obtain a film selectively permeable to carbon dioxide gas comprising chitosan according to the present invention, the above-mentioned chitosan acid salt solution is coated on a substrate in the form of a film, sheet, plate or the like and is dried (for example, at 30 to 150xc2x0 C., for 1 second to 30 hours) together with the substrate film to obtain a dry coating of chitosan acid salt. That is, the film is molded by a solvent casting method in which a chitosan acid salt solution is cast on a support (substrate) such as a metal plate, a glass plate or a plastic film and dried to form a coating, an extrusion method in which an aqueous chitosan acid salt solution of a high concentration is cast through a small slit of an extruder in the form of a film under a discharge pressure and the wet film is dried on a rotary drum or a belt, a method in which after coating the aqueous solution on a plastic film, the coated film is dried by heating or the like method. In this way, a dry chitosan acid salt coating is obtained. Of those film-forming methods, in particular, the solvent casting method (casting method, coating method) is preferably used since molded films (dry coatings) having excellent transparency can be easily obtained.
The chitosan acid salt coating is water-soluble. A film comprising water-insoluble chitosan is obtained by treating the dry coating with an alkaline aqueous solution, for example dipping it in an aqueous sodium hydroxide solution (for example, dipping it in an aqueous 1 N sodium hydroxide solution for 0.5 seconds to 48 hours), and then washing it with water (for example, in tap water for 1 second to 1 hour) to obtain a film made of chitosan which is insoluble in water. By drying this film (for example, at 30 to 200xc2x0 C. for 0.5 seconds to 1 hour), the film of the present invention can be obtained.
Here, to obtain a chitosan acid salt solution, 0.1 to 10 mass parts, preferably 0.2 to 5 mass parts, of an acid, for example, acetic acid, and 5 to 1,000 mass parts of a solvent may be mixed with 1 mass part of chitosan to dissolve chitosan for preparing it. Note that water is preferred as the solvent. A mixed solution of water and an organic solvent that is soluble in the water (for example, 2-propanol, ethanol or the like) may also be used as far as the chitosan is dissolved therein. In a case where the chitosan acid salt coating is converted back into a chitosan coating, the chitosan acid salt film is dipped in, for example, an aqueous 1 N sodium hydroxide solution at 5 to 60xc2x0 C. for 0.5 seconds to 48 hours. The chitosan coating (film) thus obtained is peeled from the substrate and can be used alone as a chitosan film. The film of chitosan alone is a film selectively permeable to carbon dioxide gas in which the ratio of the carbon dioxide gas transmission (CO2TR) to the oxygen gas transmission (O2TR), (CO2TR/O2TR), is 15 or higher, more preferably 17 or higher. Its oxygen gas transmission under the conditions of 23xc2x0 C. and 80% RH is preferably 0.1 to 2,000 (cm3/m2xc2x7dayxc2x7atm) (0.987 to 19.74xc3x97103 cm3/m2xc2x7dayxc2x7MPa), more preferably 0.1 to 1,000 (cm3/m2xc2x7dayxc2x7atm) (0.987 to 9.87xc3x97103 cm3/m2xc2x7dayxc2x7MPa). Its carbon dioxide gas transmission under the conditions of 23xc2x0 C. and 80% RH is preferably 3to 60,000 (cm3/m2xc2x7dayxc2x7atm) (29.6 to 5.922xc3x97105 cm3/m2xc2x7dayxc2x7MPa), more preferably 3 to 30,000 (cm3/m2xc2x7dayxc2x7atm) (29.61 to 29.6xc3x97104 cm3/m2xc2x7dayxc2x7MPa). When using a film of chitosan singly, the film thickness is preferably 3 to 500 xcexcm, more preferably 10 to 400 xcexcm. Furthermore, by adjusting the thickness of chitosan film, its (CO2TR) may be set in the aforementioned range of 3 to 60,000 (cm3/m2xc2x7dayxc2x7atm) (29.61 to 5.922xc3x97105 cm3/m2xc2x7dayxc2x7MPa).
As for the substrate as a support used for producing a film composed of chitosan alone or a film used together with a substrate layer, at least one material selected from a film comprising a thermoplastic resin, a film comprising a thermosetting resin, paper, fabric, nonwoven fabric, metal porous material and an inorganic sintered porous material may be used. The substrate layer may be selected as appropriate in consideration of its application purpose, the fact that the gas transmission of the substrate layer should not extremely inhibit selective carbon dioxide gas permeability of the laminate film, and so on.
The raw material of a film comprising a thermoplastic resin is not particularly limited and includes, for example, polyester, polyolefin, polystyrene, polyvinyl chloride, polyvinylidene chloride, polyamide, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, etc., and preferably polyester, polyvinylidene fluoride, polyolefin and polyamide. The polyester includes polyethylene terephthalate (PET), polyethylene isophthalate, polybutylene terephthalate, poly(1,4-cyclohexylenedimethylene terephthalate), polyethylene 2,6-naphthalenedicarboxylate, etc., and copolymers and blends of these with each other, or blends of these with small amounts of other resins and so on.
The polyolefin includes homopolymers or copolymers of olefins, copolymers of olefins with other copolymerizable monomers, for example, vinyl-based monomers and modified polymers of these. Specific examples thereof include high density polyethylene, low density polyethylene (hereinafter, abbreviated as xe2x80x9cLDPExe2x80x9d), linear low density polyethylene (hereinafter, abbreviated as xe2x80x9cLLDPExe2x80x9d), linear very low density polyethylene (hereinafter, abbreviated as xe2x80x9cVLDPExe2x80x9d), ethylene/xcex1-olefin copolymers using a single site catalyst (for example, ssc-VLDPE, ssc-LLDPE, etc.), polypropylene, ethylene/propylene copolymer, poly(4-methylpentene-1) (hereinafter, abbreviated as xe2x80x9cPMPxe2x80x9d), ionomer resin, ethylene/vinyl acetate copolymer (hereinafter, abbreviated as xe2x80x9cEVAxe2x80x9d), etylene/acrylic acid copolymer, ethylene/methacrylic acid copolymer (hereinafter, abbreviated as xe2x80x9cEMAAxe2x80x9d), ethylene/methyl acrylate copolymer, ethylene/methyl methacrylate copolymer, modified polyolefin (for example, reaction products between homopolymers or copolymers of olefins and unsaturated carboxylic acid such as maleic acid or fumaric acid or acid anhydride or ester or metal salt thereof) and so on. The above-mentioned polyolefin may be used singly or two or more of them may be used in admixture. They may be stretched or unstretched. Among these, PMP, VLDPE, EVA, EMAA, LLDPE, ethylene/acrylic acid copolymer and the like are preferable in consideration of physical property of the obtained laminate.
As one preferred embodiment of the present invention, there is provided an invention of a film selectively permeable to carbon dioxide gas, which consists of a laminate film including at least three layers composed of an outer layer, an intermediate layer and an inner layer, in which the outer layer and the inner layer include a thermoplastic resin and the intermediate layer includes a film made of chitosan, and in which the (CO2TR/O2TR) as the laminate film is 10 or higher.
In a laminate film in which the intermediate layer includes the film made of chitosan, the aforementioned substrate may be used as it is as the material for constituting the outer layer and the inner layer.
The substrate may be used as a support when chitosan is used alone or when chitosan is used together with the substrate. However, it may be used by itself as an outer layer or as an inner layer of the laminate. When it is used as a laminate film, it is preferable that the outer layer and the inner layer of the laminate film comprise a thermoplastic resin. For example, in the case of a laminate obtained by coating chitosan acid salt on a PMP film as a substrate and converting it back to chitosan with an alkali, the PMP film layer constitutes the outer layer or the inner layer of the laminate. For example, by sticking a stretched polyethylene film separately provided having coated on the surface thereof an adhesive and the previously obtained laminate film to each other so that the adhesive side of the stretched polyethylene film and the chitosan side of the laminate film can contact to each other, a laminate film of PMP layer/chitosan layer/adhesive layer/stretched polyethylene layer is obtained. When particularly strong adhesion is necessary between the PMP layer and the chitosan layer, an adhesive is coated on the PMP film and a chitosan acid salt solution is coated thereon, followed by alkali treatment to convert the chitosan acid salt solution to chitosan, and then the aforementioned stretched polyethylene film is applied on its adhesive side to obtain a laminate film of PMP layer/adhesive layer/chitosan layer/adhesive layer/stretched polyethylene layer. In the present invention, it is preferable that the intermediate layer is a laminate film including the aforementioned chitosan film layer and the thickness of the chitosan layer is 0.2 to 50 xcexcm, more preferably 0.5 to 20 xcexcm. In some cases, either outer layer or inner layer may be provided with at least one layer selected from paper, fabric, nonwoven fabric, polyolefin-based nonwoven fabric, porous polyolefin layer, porous polyester layer, and porous polyamide layer for the purpose of reinforcement.
For the inner layer (the layer contacting the object to be packaged) of a laminate film, it is preferable that a heat sealable, high frequency sealable, or supersonic sealable material (sealant) be used in consideration of a case where films are to be heat-bonded when bags and the like are produced from the laminate. The heat sealable resin includes, for example, polyolefins such as low density polyethylene, linear low density polyethylene, high density polyethylene, ethylene/vinyl acetate copolymer, ethylene-based copolymer obtained by using a metallocene catalyst, propylene-based copolymer obtained by using a metallocene catalyst, unstretched polypropylene, ethylene/acrylic acid copolymer, ethylene/acrylic acid salt copolymer, and ethylene/ethyl acrylate copolymer, nylon copolymers such as a nylon 6xc2x766 copolymer and a nylon 6xc2x712 copolymer, and the like. The high frequency sealable resin includes polyvinyl chloride, polyvinylidene chloride, a nylon 6, a nylon 66 and the like. The materials of the outer and inner layer may be the same or different.
Although the thickness of respective layers of the laminate film is not particularly limited except for the thickness of the chitosan film that is included in the intermediate layer, it is preferable that the laminate film as a whole has a thickness of 30 to 500 xcexcm, more preferably 50 to 200 xcexcm. Furthermore, in a case where the inner layer is used as a seal layer, it is preferable that the thickness of the inner layer is 10 to 100 xcexcm, more preferably 15 to 80 xcexcm in consideration of the seal strength and gas transmission.
The laminate film of the present invention has oxygen gas transmission at 23xc2x0 C. at 80% RH of preferably 10 to 5,000 (cm3/m2xc2x7dayxc2x7atm) (98.7 to 49.35xc3x97103 cm3/m2xc2x7dayxc2x7MPa), more preferably 20 to 3,000 (cm3/m2xc2x7dayxc2x7atm) (197.4 to 29.61xc3x97103 cm3/m2xc2x7dayxc2x7MPa). Its carbon dioxide gas transmission under the conditions of 23xc2x0 C. and 80% RH is preferably 300 to 150,000 (cm3/m2xc2x7dayxc2x7atm) (29.6xc3x97102 to 14.9xc3x97105 cm3/m2xc2x7dayxc2x7MPa), more preferably 600 to 90,000 (cm3/m2xc2x7dayxc2x7atm) (59.22xc3x97102 to 88.83xc3x97104 cm3/m2xc2x7dayxc2x7MPa). The ratio (CO2TR/O2TR) as the laminate film is 10 or higher, preferably 12 or higher.
The water vapor transmission rate at 40xc2x0 C. at 90% RH is preferably 1 to 100 g/m2xc2x7day, more preferably 1 to 30 g/m2xc2x7day from the viewpoint of prevention of discoloration and weight loss of foods.
Since the chitosan film of the present invention alone or a laminate film including the chitosan film of the present invention as an intermediate layer has a ratio (CO2TR/O2TR) of 10 or higher, it is effective to use the film for packaging foods which tend to generate carbon dioxide gas and to which oxygen is harmful. Foods are not particularly limited as far as they have the above-mentioned properties, namely, tendency to generate carbon dioxide gas and harmfulness of oxygen gas thereto, and the foods include coffee, cheeses, bean pastes, pickles, root crops, mushrooms, leaf-stem crops, fruit vegetables, fruits and the like.
The coffee includes bean-like and pulverized-like ones before or after roasting. The cheeses include, for example, natural cheeses such as Emmenthaler, Cheddar, Gruyere, and Gouda, or process cheeses containing these. The bean pastes include, for example, white bean paste, red bean paste, raw bean paste, etc., and in particular bean paste in products in which fermentation continuously proceeds and the like. The pickles include, for example, brine pickles of scallion, ginger, pickled plum, Chinese cabbage, etc., pickles in sake lees such as narazuke pickles and wasabi (Japanese horseradish) pickles, koji pickles such as bettarazuke pickles, bean paste pickles such as wild vegetables and radish, vinegar pickles such as senmaizuke (turnip pickles), mustard pickles such as eggplant mustard pickles, mash pickles such as cucumber pickles, and fermented pickles such as kimchee. The root crops include, for example, yam, yamato potato, lotus root, potato, ginger, burdock, taro, etc. The mushrooms include shiitake, nameko, enokitake, shimeji, mushroom, bean sprouts, etc. The leaf-stem crops include, for example, spinach, asparagus, garlic, cabbage, Chinese cabbage, lettuce, scallion, cauliflower, etc. The fruit vegetables include, for example, green soybeans and field peas. The fruits include, for example, orange, apple, grape, peach, pear, persimmon, plum, chestnut, almond, etc. In addition, cut flowers may also be included.
Food packaging films comprising the film selectively permeable to carbon dioxide gas according to the present invention can be used in ripening and storage upon manufacture or production of these foods, storage of products in the process of their distribution, and exhibition etc. as necessary to obtain objective packages.